1. Field of the Invention
The present invention relates to high power traveling wave tubes (TWTs) having gyrating electron beams, and more particularly, to a harmonic gyro-TWT having a multipole field that propagates at a frequency lower than all other possible waveguide modes.
2. Description of Related Art
Electron cyclotron resonance devices, also known as "fast-wave" tubes, refer to a particular class of high power microwave tubes. As in other "slow-wave" microwave tubes, an electron beam that passes through the device interacts with a propagating electromagnetic wave to produce amplification of the wave. Slow-wave tubes include a radio-frequency (RF) structure configured so that the phase velocity of the electromagnetic wave is slowed to the electron beam velocity. In contrast, the RF structure of fast-wave tubes comprises a smooth waveguide or a large resonator in which no attempt is made to reduce the velocity of the propagating electromagnetic wave through the device. Instead, the electron beam is injected into the electromagnetic field in a manner such that interaction can take place.
In cyclotron resonance devices, such as gyrotrons, gyro klystrons, gyro-TWTs, and gyro backward wave oscillators (BWOs), electrons of the electron beam have substantial motion perpendicular to the axis of the beam and a focusing magnetic field, and thus rotate in a helical path around a central axis. The electrons interact with RF fields perpendicular to the magnetic focusing field. As the electrons rotate and the fields alternate in synchronism, there is a cumulative interaction in which some electrons gain energy and other electrons lose energy. The electrons that gain energy undergo a relativistic-mass increase, and the ones that lose energy undergo a relativistic-mass decrease. Thus, the cyclotron frequencies of the electrons decrease or increase respectively, and as a result, the electrons gather into rod-like bunches that rotate around the axis of the helix.
The rotating electron bunches interact with a multipole field of order equal to twice the number of the desired harmonic interaction. For example, there will be a strong interaction between a TE.sub.31 mode in which the waveguide field which has six cusps of electric field and a beam rotating at one-third the frequency of the electromagnetic wave in the waveguide, assuming that the diameter of the axis-encircling electron beam is sufficiently large, or the transverse energy of the electron beam is large with respect to its axial energy. For example, an 80 kilovolt beam travels at a velocity about equal to half the velocity of light, and therefore, the diameter of an axis-encircling electron beam in which the transverse energy corresponds to 80 kilovolts will have a diameter about half of the diameter of a waveguide propagating the TE.sub.31 mode.
While an interaction between such a beam and such a waveguide mode may be desirable for certain applications, it may not always be desirable to generate quite as much power as such a beam would be capable of producing. If the energy of the beam is reduced, however, the diameter of the beam will also reduce correspondingly, and the beam will not encounter strong fields near the axis of a TE.sub.31 mode. The use of fins projecting inward from the wall of the circular waveguide has been suggested as a way to strengthen the field at such small diameters and to improve the interaction impedance.
Nevertheless, the fins further complicate the already complex mode structure of the waveguide. The use of fins will cause the waveguide to propagate in numerous modes all having strong fields. Each of the pie-shaped resonators defined between the fins can support a different mode with high fields between the ends of the fins when the fins are about a quarter of a wavelength long. Many of the modes that exist in the waveguide can be represented as differently phased combinations of the modes that exist in the individual pie-shaped resonators. That is, if the phase of the electric field in all of the pie-shaped resonators is in the same direction, a field pattern is defined that is equivalent to the TE.sub.01 circular electric mode. The TE.sub.31 mode described above is the mode that would exist if the phase in each of the pie-shaped resonators alternates from one resonator to the next. If the phase of three of the pie-shaped resonators on one side of the waveguide is in one direction and the three pie-shaped resonators on the other side is in the opposite direction, this mode would correspond to the TEll mode in a circular waveguide.
Since the resonant frequency of an individual one of the pie-shaped resonators is determined by its fin length, all of the various waveguide modes that are perturbed by the fins will now occur at frequencies very close to one another. For example, if the electron beam is a little-off center, a TE.sub.21 mode may be excited in the waveguide. Numerous other interactions are possible if the beam is modulated with modes in which the electric field is axial (TM modes) and still get azimuthal bunching which can interact with TE modes or even the radial fields of TM modes.
Accordingly, it would be desirable to provide a waveguide for a harmonic gyro-TWT that would not have the problem of nearby propagating modes that interact with the beam simultaneously with the desired mode. More specifically, it would be desirable to provide within a gyro-TWT a mode having an electric field in a high-order multipole configuration that propagates at a frequency lower than that of all of other possible modes of the waveguide.